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Y656
DESCRIPTION
Title of Invention: Steel Sheet for Hot Stamping Use,
Method of Production of Same, and Method of Production of
5 High Strength Part
Technical Field
[0001] The present invention to steel sheet for hot
stamping use which is excellent in delayed fracture
10 resistance, a method of production of the same, and a
high strength part which is formed by hot stamping using
this steel sheet. In particular, it relates to a method
of production of a high strength part which is used for a
structural part of an automobile.
15 Background Art
[0002] In recent years, reduction of the weight of
automobiles has been strongly demanded from the viewpoint
of the global environment. In automobile bodies, for
example, pillars, door impact beams, bumper beams, and
20 other structural parts for automobiles, high strength
steel sheet is being used to reduce the thickness of
steel sheet to try to lighten the weight. For this
reason, the strength of steel sheet is being raised. In
particular, high strength steel sheet with a tensile
25 strength (TS) over 1000 MPa is being developed, but
higher strength of steel sheet leads to a drop in the
workability and press formability at the time of
production of a part. In particular, it becomes more
difficult to ensure product precision due to springback
30 etc.
[0003] To solve these problems, in recent years, as a
technique for simultaneously satisfying higher strength
and workability of the steel sheet and product precision,
the hot stamping method (press quenching method) has come
35 to be used as a practical method. For example, this is
disclosed in PLT 1. This heats steel sheet to an
approximately 900°C or so austenite region, then press
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forms it hot and, at the time of press forming, brings it
into contact with an ordinary temperature die set to
quench it and thereby obtain a high strength material.
Due to this hot stamping method, the residual stress
5 which is introduced at the time of press forming is also
reduced, so the inconveniences of fracture, poor shape
freezing, etc. which become problems in high strength
steel sheet with a TS of over 1180 MPa are suppressed and
production of parts with relative good product precision
10 becomes possible.
[0004] In the high strength steel sheet which is used
for automobiles etc., the above-mentioned problems become
more serious the higher the strength. Further, in
particular, in high strength materials of over 1000 MPa,
15 as has been known in the past, there is the inherent
problem of hydrogen embrittlement (also called "season
cracking" or "delayed fracture"). In the case of steel
sheet for hot pressing use, while the residual stress due
to pressing at a high temperature is small, hydrogen
20 penetrates the steel at the time of heating before
pressing and the susceptibility to hydrogen embrittlement
becomes higher due to the residual stress after pressing.
[0005] As the method of preventing cracking due to
delayed fracture, there is the method of controlling the
25 heating atmosphere at the time of hot stamping. For
example, PLT 2 proposes the method of making the hydrogen
concentration in the heating atmosphere of the hot
stamping 6 vol% or less and making the dew-point 10°C.
This relates to a method of control of the heating
30 atmosphere of hot stamping. That is, by controlling the
hydrogen concentration and the condensation point, the
penetration of external hydrogen into the steel sheet
during heating is suppressed. Therefore, this does not
improve the steel sheet itself. It can only be applied in
35 hot stamping which has a system for controlling the
atmosphere.
[0006] In addition, as the steel sheet for hot
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stamping use, there is known steel sheet which traps the
hydrogen which penetrates the steel sheet and thereby
prevents delayed fracture. For example, PLT 3 proposes
steel sheet for hot stamping use which improves the
5 delayed fracture resistance. This art incorporates
average particle size 0.01 to 5.0 \im range Mg oxides,
sulfides, composite crystals, and composite precipitates,
e.g. one or more composite oxides among them, into the
steel in an amount of lxlO2 to lxlO7 per square mm. These
10 oxides and composite crystals and composite precipitates
having these as nuclei act as hydrogen trap sites to
thereby improve the delayed fracture resistance.
[0007] Further, as similar art, PLT 4 discloses the
art of producing high strength thin-gauge steel sheet
15 which is excellent in hydrogen embrittlement resistance
characterized by making bainite or martensite the biggest
phases in terms of area rate, making one or more of Nb,
V, Cr, Ti, and Mo oxides, sulfides, nitrides, composite
crystals, and composite precipitates in the particles
20 satisfy an average particle size "d": 0.001 to 5.0 |am, a
density p: 100 to lxl013/mm2, and a ratio of standard
deviation a of average particle size and average particle
size "d": cr/d<1.0, and by having a tensile strength of
980 MPa or more.
25 [0008] Furthermore, in steel sheet for enameling use,
to improve the fishscale susceptibility, it is known that
it is effective to form voids in the steel sheet to trap
the hydrogen. PLT 5 proposes to form Fe-Nb-Mn-based
composite oxides in steel sheet and increase the
30 segregation of Nb and Mn in the oxides so as to raise the
hydrogen trapping ability. However, the art which is
described in PLT 5 is art which assumes steel sheet for
enameling use which has a small C (carbon) content
(usually 0.01 mass% or less). In large C content high
35 strength steel sheet (C of 0.05 mass% or more) such as
steel sheet for automobile use, the oxidizing action of C
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cannot be ignored. Therefore, this cannot be simply
applied.
[0009] Further, the amount of hydrogen problematic in
steel sheet for enameling use is a high concentration of
5 10 to 100 ppm, while with high strength steel sheet, an
amount of hydrogen of a very low concentration of 1 to 3
ppm is considered a problem.
Therefore, the art which is described in PLT 5 cannot be
applied as is to large C content high strength steel
10 sheet.
[0010] To apply these arts to large C (carbon) content
high strength steel materials, suitable control of the
size (average particle size) and presence (density) of
the oxides etc. present in the steel sheet is an
15 important requirement. However, strict control to give a
particle size and density which are effective as hydrogen
trap sites and which do not form starting points of
coarse cracks is not technically easy.
20 Citations List
Patent Literature
[0011] PLT 1: Japanese Patent Publication No. 10-
96031A
PLT 2: Japanese Patent Publication No. 2006-51543A
25 PLT 3: Japanese Patent Publication No. 2006-9116A
PLT 4: Japanese Patent Publication No. 2005-68548A
PLT 5: WO2008/038474A
Summary of Invention
30 Technical Problem
[0012] Above, the state of the art regarding measures
against delayed fracture due to hydrogen embrittlement of
hot stamped steel sheet was explained. The problem is
that there is at the present point of time no art which
35 suppresses delayed fracture due to hydrogen embrittlement
when hot stamping large C content high strength steel
sheet.
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[0013] Therefore, an object of the present invention
is to provide steel sheet for hot stamping use which is
excellent in part strength after hot stamping and delayed
fracture resistance comprised of large C content high
5 strength steel sheet in which strength is secured while
effective hydrogen traps are formed in the steel
material, a method of production of the same, and a
method of production of a hot stamped high strength part.
Solution to Problem
10 [0014] The inventors took note of the fact that, to
improve steel sheet for hot stamping use in delayed
fracture resistance, trapping the hydrogen which
penetrates the steel sheet is effective and engaged in
intensive research based on that. As a result, they
15 discovered that it is possible to cause the formation of
Fe-Mn-based composite oxides in steel sheet and trap the
hydrogen at the interfaces of the composite oxides and
the matrix steel and thereby completed the present
invention.
20 [0015] In large C content high strength steel sheet,
usually the inclusions of metal oxides become defects.
For this reason, as much as possible, the oxygen in the
steel is removed and formation of metal oxides is
suppressed. Therefore, in addition to adding Al and other
25 deoxidizing elements, the concentration of oxygen is
reduced at the stage of molten steel.
[0016] However, to cause the formation of Fe-Mn-based
composite oxides in the steel like in the present
invention, it is necessary to leave oxygen in the steel
30 to a certain extent. Further, C itself has a deoxidizing
action, so in general, with large C content steel sheet,
the oxygen in the steel ends up becoming small in amount.
[0017] Therefore, the inventors discovered that by
reducing the concentration of Al in the steel sheet,
35 weakening the deoxidizing effect, and securing a
concentration of oxygen in the steel, it is possible to
cause the formation of composite oxides even in large C
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content steel sheet.
[0018] Further, they discovered that to raise the
hydrogen trapping effect of composite oxides, it is
effective to crush the composite oxides and increase
5 their surface area. They discovered that by crushing and
making the composite oxides finer, their effect as
defects falls and this leads to an improvement in the
performance of the steel sheet.
Furthermore, they learned that if there are voids around
10 the composite oxides, the hydrogen trapping effect is
improved more.
[0019] The inventors engaged in intensive studies on
the method of production for the above.
They learned that large C content molten steel is high in
15 viscosity, so Fe-Mn-based composite oxides have
difficulty rising and steel Fe-Mn-based composite oxides
can be easily formed in the steel.
[0020] Further, it was learned that by rolling (hot
rolling or further cold rolling) a slab comprised of
20 steel in which Fe-Mn composite oxides are formed, the
composite oxides can be stretched and crushed. In this
way, they discovered that it is possible to efficiently
form hydrogen trap sites in steel sheet which do not
become starting points of cracks. Further, they
25 discovered that it is possible to form effective voids in
a similar process. The present invention was completed
based on these discoveries. The present invention has as
its gist the following:
[0021] (1) Steel sheet for hot stamping use which is
30 comprised of chemical ingredients which contain, by
mass%,
C: 0.05 to 0.40%,
Si: 0.001 to 0.02%,
Mn: 0.1 to 3%,
35 Al: 0.0002 to 0.005%,
Ti: 0.0005 to 0.01%,
0: 0.003 to 0.03%,
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one or more of Cr and Mo in a total of 0.005 to 2%, and
a balance of Fe and unavoidable impurities,
wherein the steel sheet contains average diameter 0.1 to
15 |am Fe-Mn-based composite oxide particles dispersed in
5 the steel sheet.
Note that, S, P, and N are unavoidable impurities, but
are restricted to the following contents:
S: 0.02% or less,
P: 0.03% or less,
10 N: 0.01% or less,
(2) The steel sheet for hot stamping use as set forth
in (1) which further contains, by mass%, the ingredients
which are included in one or more groups among the three
groups of (a) to (c):
15 (a) B: 0.0005 to 0.01%;
(b) one or more of Nb, V, W, and Co in a total of 0.005
to 1%; and
(c) one or more of Ni and Cu in a total of 0.005 to 2%.
(3) The steel sheet for hot stamping use as set forth
20 in (1) or (2), wherein there are voids around the
composite oxide particles.
(4) The steel sheet for hot stamping use as set forth in
(1) or (2), wherein the voids around the composite oxide
particles have average sizes of 10 to 100% of the average
25 size of the composite oxide particles.
(5) The steel sheet for hot stamping use as set forth in
(1) or (2), wherein the steel sheet is plated by any of
aluminum plating, zinc-aluminum plating, and zinc
plating.
30 (6) A method of production of steel sheet for hot
stamping use comprising hot rolling a slab of chemical
ingredients set forth in (1) or (2) in which rough
rolling the slab by a rolling rate of 70% or more and
final rolling the slab by a rolling rate of 70% or more.
35 (7) The method of production of steel sheet for hot
stamping use as set forth in (6), further comprising
pickling the hot rolled steel sheet which was obtained by
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hot rolling and cold rolling the steel sheet by a rolling
rate of 30% or more.
(8) The method of production of steel sheet for hot
stamping use as set forth in (7), further comprising
5 annealing the cold rolled steel sheet which was obtained
by cold rolling.
(9) A method of production of a high strength part using
the steel sheet for hot stamping use comprising heating
the steel sheet as set forth in (1) or (2) to a
10 temperature of austenite region of the Ac3 or higher, then
starting to form the steel sheet by a die, and cooling
the steel sheet in the die after forming to quench.
Advantageous Effects of Invention
15 [0022] The high strength steel sheet for hot stamping
use of the present invention stretches and crushes
composite oxides to thereby form composite oxide
particles and their surrounding voids which are effective
as hydrogen trap sites. Due to this, there is no need to
20 strictly control the size (average particle size) and
state of presence (density) of oxides etc. like in the
past and it is possible to provide steel sheet which is
excellent in delayed fracture characteristics. If using a
member which is produced from the steel sheet of the
25 present invention, it is considered possible to greatly
contribute to the lighter weight and greater safety of
automobiles. The contribution to industry is great.
Brief Description of the Drawings
[0023] FIG. 1 is a schematic view which shows the
30 state where coarse composite oxides are stretched and
crushed and many crushed voids (hydrogen trapping
ability) are formed in the steel sheet.
FIG. 2 is a schematic view which shows the state where
coarse oxides are stretched and crushed and few crushed
35 voids (hydrogen trapping ability) are formed in the steel
sheet.
FIG. 3 is a schematic view which shows that crushed voids
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are not formed when there are fine oxides present.
FIG. 4 is a cross-sectional view of the shape of a die
set which is used in the examples.
FIG. 5 is a view which shows the shape of a punch which
5 is used in the examples as seen from the top.
FIG. 6 is a view which shows the shape of a die which is
used in the examples as seen from the bottom.
FIG. 7 is a schematic view of a hot stamped part.
FIG. 8 is a view which shows the shape of a test part for
10 evaluation of delayed fracture resistance as seen from
the top.
Description of Embodiments
[0024] Below, the present invention will be explained
15 in detail.
[0025] The fact that delayed fracture occurs due to
the diffusible hydrogen which penetrates the steel sheet
from the outside environment and diffuses in the steel
sheet at room temperature is already known. Therefore, if
20 able to trap hydrogen which penetrates from the outside
environment at some part inside the steel sheet, it would
become possible to render the hydrogen harmless and
delayed fracture would be suppressed.
[0026] The inventors discovered that by casting a
25 slab comprised of steel in which Fe-Mn-based composite
oxides are formed in the steelmaking process and by hot
rolling and cold rolling the slab to stretch and crush
the composite oxides, it is possible to form fine voids
between the finely crushed Fe-Mn-based composite oxide
30 particles, that the voids are effective as hydrogen trap
sites, that diffusible hydrogen, which is believed to be
the cause of delayed fracture, is trapped at those parts,
and the susceptibility to delayed fracture falls.
Furthermore, the inventors discovered that these voids
35 were of sizes and shapes by which they did not easily
become starting points of cracks and attempted to apply
the steel for a hot stamping material in which strength
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is demanded.
[0027] First, the reasons for limiting the strength of
a part after hot stamping of the present invention and
the ingredients of steel sheet for hot stamping use which
5 is excellent in delayed fracture resistance to
predetermined ranges will be explained. Here, for the
ingredients, % means mass%.
[0028] C: 0.05 to 0.40%
C is an element which is added to make the structure
10 after cooling martensite and secure the material quality.
To improve the strength, 0.05% or more of C is necessary,
but if the C content exceeds 0.40%, the strength at the
time of deformation upon impact and the weldability
deteriorate, so C was made 0.05 to 0.40%. From the
15 viewpoint of the strength, furthermore, the C content is
preferably made 0.15% or more, more preferably is made
0.2% or more.
[0029] Further, from the viewpoint of the
deterioration of the strength at the time of deformation
20 upon impact or the weldability and the effect of
deoxidation by C, the C content is preferably 0.35% or
less, more preferably 0.3% or less.
[0030] Si: 0.001 to 0.02%
Si acts as a deoxidizing element. The present invention
25 requires that a certain amount or more of oxides be
secured, so Si, which reduces the oxygen content, is
limited to 0.02% or less. To obtain the amount of
effective oxides, the Si content is made 0.015% or less,
more preferably 0.01% or less. The lower limit of the Si
30 content is not particularly an issue, but due to the time
and expense involved in removing Si, 0.001% is made the
lower limit.
[0031] Mn: 0.1 to 3%
Mn is an element which affects the hot stampability and
35 hardenability and is effective for raising the strength
of the steel sheet. Further, Mn, by addition, forms Fe-Mn
composite oxides, so is an important ingredient in the
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present invention. These composite oxides form trap sites
for the hydrogen which causes delayed fracture. For this
reason, addition of Mn is effective for improvement of
the delayed fracture resistance.
5 [0032] Further, the formed composite oxides are fine
in size, so are effective for suppressing the formation
of coarse cracks at the punched surfaces. To form oxides
and utilize Mn to the maximum extent as hydrogen trap
sites, it is sufficient to proactively add Mn since
10 addition facilitates control of the oxide composition. If
Mn is less than 0.1%, this effect cannot be obtained. For
this reason, the Mn content may be made 0.1% or more. To
reliably obtain this effect, the Mn content is preferably
made 0.5% or more. Furthermore, 1.30% or more is more
15 preferable.
[0033] Further, if the Mn content exceeds 3.0%, the Mn
assists co-segregation with P and S, invites a drop in
the toughness, and lowers the delayed fracture
resistance. For this reason, the Mn content should be
20 made 3% or less. More preferably, the Mn content may be
made 2.0% or less, more preferably 1.50% or less.
[0034] S: 0.02% or less
S is contained as an unavoidable impurity. If contained
in excess, it degrades the workability, becomes a cause
25 of deterioration of toughness, and lowers the delayed
fracture resistance. For this reason, the smaller the S,
the better. As the allowable range, the content is
defined as 0.02% or less. Preferably, the content should
be made 0.01% or less. Furthermore, by limiting the S
30 content to 0.005% or less, the impact characteristics are
strikingly improved.
[0035] P: 0.03% or less
P is an element which is contained as an unavoidable
impurity and has a detrimental effect on toughness when
35 excessively added. It lowers the delayed fracture
resistance. For this reason, the less the P, the better.
As the allowable range, the content is limited to 0.03%
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or less. Furthermore, 0.025% or less is preferable.
Furthermore, if 0.02% or less, the effect of improvement
of the delayed fracture resistance is large.
[0036] Al: 0.0002 to 0.005%
5 Al is an element which is required for use as a
deoxidizing material of molten steel. The present
invention requires that a certain amount or more of
oxides be secured, so if Al, who has a deoxidizing
effect, is over 0.005%, the amount of oxides for
10 improving the delayed fracture resistance cannot be
secured. For this reason, the upper limit was made
0.005%. If considering a margin of safety, the Al content
is preferably made 0.004% or less, more preferably is
made 0.003% or less. Further, the lower limit is not
15 particularly set, but removing Al involves time and
expense, so 0.0002% or more is practical.
[0037] Ti: 0.0005 to 0.01% or less
Ti is also a deoxidizing element. The lower limit is not
particularly set, but removing Ti involves time and
20 expense, so the content is sufficiently made 0.0005% or
more, preferably 0.001% or more. On the other hand,
addition of a large amount reduces the oxides which
improve the delayed fracture resistance, so the upper
limit was made 0.01%. Furthermore, 0.008% or less is
25 preferable. Furthermore, if 0.006% or less, the effect of
improvement of the delayed fracture resistance is large.
[0038] N: 0.01% or less
If N is over 0.01%, the nitrides coarsen and the
dissolved N causes hardening upon ageing, whereby a
30 tendency for the toughness to deteriorate is seen. For
this reason, the smaller the N the better. As the
allowable range of N, the content is limited to 0.01% or
less in range. Preferably, it is made 0.008% or less. If
0.006% or less, it is possible to suppress the
35 deterioration of toughness, so this is preferable.
[0039] One or both of Cr and Mo in total of 0.005 to
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Cr and Mo are both elements which improve the
hardenability. Further, they have the effect of causing
precipitation of M23C6 type carbides in the matrix and
have the action of raising the strength and refining the
5 carbides. For this reason, one or both of Cr and Mo are
added in a total of 0.005 to 2%. If less than 0.005%,
these effects cannot be sufficiently expected. More
preferably, the content should be made 0.01% or more.
Furthermore, if 0.05% or more, the effect becomes
10 remarkable. Further, if exceeding 2% in total, the yield
strength excessively rises, the toughness is degraded,
and the delayed fracture resistance is lowered. If
possible, from the viewpoint of the delayed fracture
resistance, the content is more preferably made 1.5% or
15 less.
[0040] (0: 0.003 to 0.03%)
0 is an element which is required for forming Fe-Mn
composite oxides in the present invention. Inclusion of
0.003 to 0.03% is necessary. If less than 0.003%, a
20 sufficient amount of Fe-Mn composite oxides cannot be
formed. From the viewpoint of forming Fe-Mn composite
oxides, 0.005% or more is preferable. On the other hand,
if including over 0.03%, the cast slab ends up with
blowholes and other internal defects, so the upper limit
25 was made 0.03%. From the viewpoint of internal defects,
less is better. An 0 content of 0.02% or less is
preferable. If possible, if 0.015% or less, the defects
remarkable decrease.
[0041] B: 0.0005 to 0.01%
30 B is an element which is effective for improving the
hardenability. To make this effect more effective,
addition of 0.0005% or more is necessary. To make this
effect more reliable, 0.001% or more is preferable.
Furthermore, 0.0015% or more is more preferable. On the
35 other hand, even if excessively added, the effect becomes
saturated, so 0.01% was made the upper limit. Seen from
the viewpoint of cost versus effect, 0.008% or less is
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preferable. If possible, 0.005% or less is more
preferable.
[0042] One or more of Nb, V, W, and Co in total of
0.005 to 1%
5 Nb, V, W, and Co are carbide-forming elements. They form
precipitates to secure the strength of the hot stamped
and quenched member. Furthermore, these are necessary
elements which are contained in the Fe-Mn-based composite
oxides, act as hydrogen trap sites which are effective
10 for improvement of the delayed fracture resistance, and
improve the delayed fracture resistance. One or more of
these elements may be added. If the amounts of addition
exceed a total of over 1%, the rise in the yield strength
excessively increases. For this reason, 0.7% or less is
15 more preferable. If possible, 0.5% or less is still more
preferable. On the other hand, if less than 0.005%, the
improvement in strength and the effect as a hydrogen trap
site become difficult to obtain. From the viewpoint of
reliably obtaining this effect, 0.01% or more is
20 preferable.
[0043] One or both of Ni and Cu in total of 0.005 to
2%
Ni and Cu are both elements which improve the strength
and toughness, but if added in a total of over 2%, the
25 castability falls, so the upper limit is made 2%. From
the viewpoint of the castability, the content may be
reduced. 1% or less is more preferable. 0.5% or less is
more preferable. On the other hand, if less than 0.005%
in total, the effect of improvement of the strength and
30 toughness are difficult to obtain, so one or both of Ni
and Cu may be added in a total of 0.005% or more. From
the viewpoint of the strength and toughness, 0.01% or
more is preferable. Furthermore, 0.02% or more is more
preferable.
35 [0044] Next, the method of production of steel sheet
for hot stamping use which is excellent in delayed
fracture resistance of the present invention will be
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explained.
[0045] In the present invention, it is possible to
smelt steel adjusted in composition of ingredients of the
present invention by the usual smelting, continuous
5 casting, and steel sheet production process. In
particular, to form the Fe-Mn-based composite oxides
characterizing the present invention, it is preferable to
add the weak deoxidizing ability elements first in the
steel smelting and casting processes. For example, by
10 adding Mn, Si, Al, etc. in that order, the effect of the
present invention can be obtained more remarkably.
[0046] The mechanism by which these steelmaking
conditions affect the properties of the invention steels
is believed to be the following: The fluctuations in
15 composition of the composite oxides of the invention
steels are mainly due to the fluctuations in composition
of the thermodynamic oxides at the time of melting and
solidifying the steels. Basically, this is realized by
utilizing the nonequilibrium state in the process of the
20 composition of oxides approaching the equilibrium state
due to the change in concentration and change in
temperature of the system. By adding a weak oxidation
ability element A first, the oxygen in the molten steel
forms coarse oxides of A, but by adding an element B with
25 a strong bonding force with oxygen after that, the
element A in the oxides of A is switched to the element
B. In the process, coarse composite oxides of A and B (AB
composite oxides) are formed. If ending up adding the
strong deoxidation ability element first, formation of a
30 composite after that becomes difficult. Not only that, a
large amount of oxides are formed together with addition
and deoxidation occurs. The large amount of oxides float
up in the molten steel make dispersion of oxides into the
steel difficult. As a result, the effect of improvement
35 of the delayed fracture resistance of the product is
reduced.
[0047] Due to such a mechanism, time is required for
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forming coarse composite oxides after addition of a weak
oxidizing element. On the other hand, if an excessively
long time ends up elapsing after addition of an element,
the composition of the A-B composite oxides becomes too
5 close to the oxides of B in the equilibrium state. Not
only does the effect of the composite oxides become
smaller, but also the oxides again float up and end up
leaving the molten steel so the effect of improvement of
the characteristics is inhibited.
10 [0048] The voids which function as hydrogen trap sites
are mainly formed in the cold rolling process after hot
rolling. That is, the Fe-Mn-based composite oxides are
crushed by the rolling whereby crushed voids are formed
around the composite oxide particles. For this reason, it
15 is important to control the shape of the composite oxides
in the hot rolling process.
[0049] In the present invention, the composite oxide
parties which are dispersed in the steel were originally
an integrated composite oxide. That is, at the time of
20 casting the molten steel finished being adjusted in
ingredients, there was a single large oxide mass, but
this is believed to be stretched, crushed, and finely
dispersed in the rolling process. Such stretching and
crushing mainly occurs in the rolling process. When the
25 temperature of the steel sheet is high (1000°C or more),
oxides are mainly stretched.
[0050] On the other hand, when the temperature of the
steel sheet is low (1000°C or less), the oxides are mainly
crushed. In such a process, if there is segregation in
30 composition in the oxides, the extent of stretching will
differ depending on the portion of the oxides and the
shape of the oxides will become complicated. Further, the
fine (thin) portions are preferentially crushed, while
the portions with large fluctuations in shape are
35 expected to be preferentially crushed due to the
concentration of deformation stress. As a result,
portions which differ in composition are efficiently
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crushed and dispersed. At the time of this crushing,
voids are sometimes formed around the composite oxide
particles. These also become hydrogen trap sites in the
steel and are believed to remarkably improve the delayed
5 fracture resistance of the hot stamped products.
[0051] The above will be explained with reference to
the figures.
[0052] FIG. 1 is a schematic view which shows the
state where coarse composite oxides are stretched and
10 crushed and a large number of crushed voids (hydrogen
trapping ability) are formed in the steel sheet. In FIG.
1, the coarse composite oxides 1 are formed by two
different types of oxides 1-1 and 1-2 as composites. The
composite oxides 1 are hot rough rolled 2 (shown by
15 arrows in FIG. 1) to stretched composite oxides 3 and the
oxides 3-1 and 3-2 are also stretched. Next, they are hot
final rolled 4 (shown by arrows in FIG. 1) and further
stretched and crushed. At this time, oxides of different
hardnesses are crushed, so numerous crushed voids 5 are
20 formed around the particles 5-1 and 5-2 of the crushed
composite oxides. These crushed voids 5 also become
hydrogen trap sites whereby the delayed fracture
resistance is improved.
[0053] As opposed to this, the case where, like in the
25 past, just coarse oxides are contained is shown in FIG.
2. The coarse oxides 6 are hot rough rolled 2 (shown by
arrows in FIG. 2) to become stretched oxides 7. Next,
they are hot final rolled 4 (shown by arrows in FIG. 1)
to be stretched and crushed. However, since these are
30 masses of oxides, the crushed oxides 8 also do not
disperse as fine composite oxide particles such as in the
present invention. Therefore, it is not possible to
obtain crushed voids 5 which are sufficient as hydrogen
trap sites.
35 [0054] FIG. 3 is a schematic view which shows that
crushed voids are not formed before hot rolling, that is,
there are fine oxides at the slab stage. If fine
- 18 -
composite oxides 6' at the slab stage such as in FIG. 3,
the fine oxides 61 are hard to stretch by rough rolling 2
(shown by arrows in FIG. 3). As a result, even with final
rolling 4 (shown by arrows in FIG. 3), the oxides are not
5 crushed that much, so crushed voids 5 which form hydrogen
trap sites become difficult to form.
[0055] Note that, while not shown, cold rolling, in
the same way as hot final rolling 4 (shown by arrows in
FIGS. 1 to 3), has the effect of further finely crushing
10 the oxides.
[0056] To efficiently trap the hydrogen, it is
desirable that the composite oxide particles uniformly
disperse in the steel sheet. Further, the interfaces
between the composite oxide particles and the matrix
15 steel become hydrogen trap sites, so the composite oxide
particles should have large specific surface areas
(surface areas per unit weight). For this reason, the
composite oxides are desirably fine. Further, from the
viewpoint of suppression of defects as well, the
20 composite oxides are desirably fine.
[0057] Furthermore, the voids which are formed around
the composite oxide particles also become smaller if the
composite oxide particles are small. Therefore, from the
viewpoint of reducing the volume of voids in the steel
25 sheet as well, the composite oxides preferably become
finer. Further, the fact that rolling enables the
composite oxides to be stretched, crushed, and made finer
is convenient since this is possible with current
processes as they are.
30 [0058] The Fe-Mn-based composite oxides which are
covered by the present invention are Fe-Mn-based
composite oxides comprised of oxides of Fe, Mn, Si, Al,
etc. joined together as composites. The composite oxides
are preferably fine in size, but if too fine in size, the
35 hydrogen trapping effect is reduced. Therefore, the
diameter of the composite oxides is preferably 0.10 (jm or
more. This is because in oxides which are smaller than
- 19 -
this range, the great feature in the characteristics of
the steel sheet of the present invention, that is, the
effect as hydrogen trap sites becomes extremely small.
Preferably, it is 0.50 |j.m or more, more preferably 1.0 |jm
5 or more, still more preferably 2.0 (im or more.
[0059] The upper limit of the diameter does not have
to be particularly limited if considering the effect of
the present invention. However, while depending on the
contained oxygen, if the coarse composite oxides become
10 greater, the number density of the composite oxides will
decrease and the hydrogen trapping effect will become
smaller. Further, too coarse oxides, as is generally
known, become starting points of cracking of the steel
sheet when working the product sheet and thereby impair
15 the workability. If considering these, the average
diameter of the composite oxides is preferably kept to 15
(Jin or less, preferably 10 jam or less, more preferably 5 ^im
or less.
[0060] The average diameter of the oxides and the
20 voids near the oxides are preferably observed by an
optical microscope or scan type electron microscope after
polishing a cross-section of the steel sheet.
Furthermore, for detailed observation, the steel sheet is
preferably used to prepare a thin film sample, then is
25 observed by a transmission type electron microscope.
Measurement of the voids is, for example, described in
JIS (Japanese Industrial Standard) G0555 "Microscopic
Test Methods of Nonmetallic Inclusions of Steel".
[0061] Similarly, when crushed voids are formed, their
30 sizes are not particularly limited. The size of a void is
a long axis of 0.1 to 5 jam for an aspect ratio of 2 to
10. However, if the crushed voids are too large, void
defects result and the characteristics of the steel
materials are degraded. Usually, the size is the size of
35 the crushed composite oxides. Therefore, the average size
of the crushed voids becomes 100% or less of the average
- 20 -
size of the composite oxides (particles). From the
viewpoint of the defects, the voids should also be small.
Preferably, they should be 80% or less. The lower limit
of the average size of voids is not particularly set.
5 Even if the average size is 0, that is, there is no void,
hydrogen trap sites are formed by the interfaces of the
composite oxides and steel.
The "average size of the voids" in the present invention
is defined as the average value of the long axes and
10 short axes of five voids.
[0062] Hot rolling, in particular rough rolling is
high in temperature, so the composite oxides also soften
and the difference in hardness from the matrix iron is
also small. That is, in the temperature region of rough
15 rolling, that is, about 1000°C or more temperature region,
there is almost no fracture of composite oxides due to
rolling and the composite oxides are stretched.
[0063] Further, if lower than 1000°C, preferably 900°C
or less, the composite oxides become difficult to
20 stretch. At the prior stage of hot final rolling,
fracture of the extent where fine cracks are formed occur
at part of the composite oxides. Furthermore, at the
final stage of hot rolling or at cold rolling, the
composite oxides are crushed starting from the fine
25 cracks which were formed. To obtain composite oxides
which are suitably stretched and simultaneously have fine
cracks and are crushed in this way, temperature control
at the time of hot rolling and control of the strain and
strain rate at different temperature regions becomes
30 necessary.
[0064] If the temperature region of the hot working is
too high, it is not possible to impart enough strain for
forming cracks to the composite oxides. Further, if too
low, the composite oxides are not stretched in state, but
35 become close to spherical shapes, so cracks are difficult
to form. Suitable stretching and reduction of thickness
is necessary for formation of cracks. For this reason, it
- 21 -
is necessary to control and impart stretching of the
composite oxides by suitable deformation at a higher
temperature in hot rolling and formation of cracks in the
low temperature region. Further, the form of the
5 composite oxides which form such cracks, as explained
above, becomes more complex when there is a difference in
concentration inside the composite oxides and a
difference in deformation ability. Efficient formation of
effective voids becomes possible.
10 [0065] The hot rolling heating temperature and coiling
temperature etc. of the hot rolling conditions can be set
as usual in the usual operating region. To sufficiently
obtain the effect of stretching the composite oxides in
hot rolling, the hot rolling heating temperature should
15 be made 1000 to 1400°C. Preferably, it should be made
1050°C or more. Due to this, hot rough rolling can be
performed at 1000°C or more and, after that, hot final
rolling can be performed at 1000°C or less. The last final
rolling temperature should be made 800°C or less.
20 Preferably, it should be made 750°C or less. Due to this,
the stretched composite oxides are increasingly crushed.
Making the coiling temperature 700°C or less is
advantageous economically.
[0066] Further, to control the form of the composite
25 oxides, the sheet is preferably rough rolled by a rolling
rate of 70% or more and final rolled by a rolling rate of
70% or more. The higher the rolling rate, the more
effective in crushing and stretching the composite
oxides, so the sheet is more preferably rough rolled by a
30 rolling rate of 75% or more. 80% or more is more
preferable. Further, it is still more preferable if the
rolling rate in final rolling is 80% or more. 90% or more
is more preferable. That is, with this rolling rate, the
composite oxides are stretched and crushed and become
35 hydrogen trap sites which are effective for improvement
of the delayed fracture resistance.
- 22 -
[0067] In hot rolling as well, composite oxide
particles which become hydrogen trap sites are obtained,
but further cold rolling enables the composite oxides to
be made finer and thereby the hydrogen trapping effect to
5 be improved. For the cold rolling to sufficiently crush
the composite oxides, the rolling rate in the cold
rolling should be made 30% or more. This is because with
a 30% or more cold rolling rate, the composite oxides are
stretched and crushed to form hydrogen trap sites which
10 are effective for improving the delayed fracture
resistance and the delayed fracture resistance is further
improved. Furthermore, 40% or more is more preferable,
while if 50% or more, the improvement in the delayed
fracture resistance becomes remarkable. In particular,
15 when deep drawing becomes necessary, it is preferable to
make the rolling rate in cold rolling 60% or more.
[0068] In the case of annealing, either the continuous
annealing method or the box annealing method which is
performed on ordinary cold rolled steel sheet may be
20 used.
[0069] When the steel sheet for hot stamping use is
used as a structural part for an automobile, it is mostly
used treated on its surface. In particular, it is mostly
used as plated steel sheet. As plated steel sheet,
25 usually aluminum plated, zinc-aluminum plated, and zinc
plated sheet are used. The steel sheet for hot stamping
use of the present invention may also be plated by
ordinary methods. For example, when applying hot dip
aluminum coating, the surface of the steel sheet should
30 be coated by 30 to 100 g/m2 or so at one side.
[0070] Further, to produce a high strength part by hot
stamping in the present invention, the steel sheet is
first heated in the austenite region, that is, to the AC3
transformation point or higher austenite region. In this
35 case, it is sufficient that the austenite region be
reached. If too high, coarsening of the particles or
oxidation will become remarkable, so this is not
- 23 -
preferred. Next, the sheet starts to be shaped by the die
set. By constraining the part after being worked by the
die set while rapidly cooling it and causing martensite
transformation for quenching, it is possible to produce a
5 high strength part.
[0071] If the cooling rate becomes slow, quenching is
no longer achieved and the target strength can no longer
be obtained, so the speed of rapid cooling from the
austenite region is made the critical cooling rate which
10 is affected by the steel ingredients or structure or
more. The cooling completion temperature is preferably
the martensite transformation completion temperature or
less.
[0072] Note that, tempering need not particularly be
15 performed, but may be performed in accordance with need
for correcting too high strength or improving the
toughness.
Examples
20 [0073] Below, examples will be used to explain the
present invention.
[0074] Example 1
Steels of the chemical ingredients which are shown in
Tables 1-1 to 1-3 and Tables 2-1 to 2-3 were cast to
25 produce slabs. Note that, Tables 2-1 to 2-3 show steel
types which have the Steel Types A, X, and AC which are
described in Table 1-1 and Table 1-2 as base steels and
have different ingredient elements which are described in
Tables 2-1 to 2-3 mixed in with them.
30 [0075] These slabs were heated to 1050 to 1350°C and
hot rolled at a finish temperature 800 to 900°C and a
coiling temperature 450 to 680°C to obtain thickness 4 mm
hot rolled steel sheets. After that, the sheets were
pickled, then were cold rolled to obtain thickness 1.6 mm
35 cold rolled steel sheet. After that, they were
continuously annealing (annealing temperature 720 to
- 24 -
830°C) . Further, parts of the cold rolled steel sheets
were hot dip galvanized (basis weight: one side 30 to 90
g/m2), hot dip galvannealed (basis weight: one side 30 to
90 g/m2), and hot dip aluminum coated (basis weight: one
5 side 30 to 100 g/m2) on a continuous hot dipping line. The
steel sheet types are shown in Tables 1-1 to 3 and 2-1 to
3. The types of steel sheets are shown below:
HR: hot rolled steel sheet, CR: cold rolled steel sheet
(annealed material), AL: hot dip aluminum coated steel
10 sheet, GI: hot dip galvanized steel sheet, and GA: hot
dip galvannealed steel sheet.
[0076] The average (arithmetic average) particle size
of the Fe-Mn composite oxides in a produced steel sheet
and the presence of crushed voids were determined by
15 polishing a cross-section of the steel sheet, then
observing it by an optical microscope or scan type
electron microscope or by a transmission type electron
microscope after preparing the sample into a thin film.
The results are shown together in Tables 1-1 to 3 and
20 Tables 2-1 to 3. The judgment criteria are shown below:
Average particle size of composite oxides:
Good: average diameter 0.1 to 15|am,
Poor: average diameter less than 0.1 (am or over 15 ^m
An average diameter of the composite oxides, as explained
25 above, of 0.1 to 15 (am was deemed as passing.
Crush voids around composite oxides:
Good: average size of voids 0.1 jum or more,
Poor: average size of voids less than 0.1 |am.
The average size of the crushed voids around the
30 composite oxides, as explained above, is preferably 0.1
um or more.
[0077] After that, these cold rolled steel sheets were
heated by a heating furnace to over the Ac3 point, that
is, the 880 to 950°C austenite region, then were hot
35 worked. For the atmosphere of the heating furnace,
combustion exhaust gas was used. The hydrogen
- 25 -
concentration in the atmosphere was 2%, while the dewpoint
was 20°C.
[0078] A cross-section of the die set shape is shown
in FIG. 4. FIG. 4 shows the shapes of a die 9 and punch
5 10. The shape of the punch when seen from above is shown
in FIG. 5. FIG. 5 shows the punch 10. The shape of the
die when seen from below is shown in FIG. 6. FIG. 6 shows
the die 9. In the die set, the shape of the die is
determined based on the punch with a clearance of the
10 sheet thickness of 1.6 mm. The blank size was made 1.6 mm
thicknessx300 mmx500 mm. The shaping conditions were made
a punch speed of 10 mm/s, a pressing force of 200 tons,
and a holding time at bottom dead center of 5 seconds. A
schematic view of the hot stamped part 11 is shown in
15 FIG. 7.
[0079] The quenching characteristic of the hot stamped
part was evaluated by polishing the cross-section,
corroding it by Nital, then observing the microstructure
by an optical microscope and determining the area rate of
20 martensite. The results of judgment are shown in Tables
1-1 to 1-3 and Tables 2-1 to 2-3. The judgment criteria
are shown below:
Good: martensite area rate 90% or more,
Fair: martensite area rate 80% or more, and
25 Poor: martensite area rate less than 80%.
A martensite area rate of 80% or more was deemed the
preferable range.
[0080] The delayed fracture resistance was evaluated
by imparting stress by piercing. The pierce hole position
30 13 at the center of the test part 12 which is shown in
FIG. 8 was pierced using a diameter 10 mm punch and using
a diameter 10.5 mm die. FIG. 8 shows the shape of the
part seen from above. FIG. 8 shows the part 12 and the
pierce whole center 13. The piercing was performed within
35 30 minutes after hot shaping. The number of parts
observed was 10. For judgment of the hydrogen
embrittlement resistance, the entire circumference of the
f - 26 -
hole was observed one week after piercing to judge the
presence of any cracks. The state was observed by a loupe
or electron microscope. The results of judgment are shown
in Tables 3. The judgment criteria are shown below:
5 Total of number of parts with fine cracks in 10 parts:
Very good: 0,
Good: 1,
Fair: less than 5,
Poor: 5 or more.
10 A number of parts with fine cracks of less than five was
judged as passing, but of course the smaller the number
the better.
[0081] As shown in Tables 1-1 to 1-3 and Tables 2-1
to 2-3, if in the scope of the present invention, it is
15 learned that it is possible to realize steel sheet which
is sufficiently strengthened by die quenching by hot
stamping and is excellent in delayed fracture resistance.
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Table 1-2
Ex.
no.
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
Steel
type
P
Q
R
S
T
U
V
W
X
Y
Z
AA
AB
AC
AD
Y
Z
AA
AB
AC
AD
Y
Z
AA
AB
AC
AD
Y
Z
Steel
sheet
type
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
AL
AL
AL
AL
AL
AL
GI
GI
GI
GI
GI
GI
GA
GA
C
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.15
0.10
0.03
0.25
0.30
0.55
0.15
0.10
0.03
0.25
0.30
0.55
0.15
0.10
0.03
0.25
0.30
0.55
0.15
0.10
Si
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
Mn
1.2
1.2
2
1.8
1.8
1.3
0.2
1.2
1.3
1.5
1.7
1.8
1.2
1
0.4
1.5
1.7
1.8
1.2
1
0.4
1.5
1.7
1.8
1.2
1
0.4
1.5
1.7
P
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
s
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
0.002
Al
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
Ti
0.001
0.04
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
0.004
N
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
0.003
Cr
1
1
0.1
0.8
0
2.5
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
Mo
0.2
0.2
0.005
0.08
0
0
3
0
Cr+
Mo
1.2
1.2
0.005
0.08
0.1
0.8
3
2.5
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0
0.03
0.0013
0.0149
0.0153
0.0148
0.0145
0.0154
0.015
0.0163
0.0183
0.0193
0.0233
0.0134
0.0121
0.0025
0.0183
0.0193
0.0233
0.0134
0.0121
0.0025
0.0183
0.0193
0.0233
0.0134
0.0121
0.0025
0.0183
0.0193
B
0.0048
0.0052
0.0048
0.0048
0.0045
0.0054
0.0043
0.0052
0.0048
0.0048
0.0045
0.0054
0.0043
0.0052
0.0048
0.0048
0.0045
0.0054
0.0043
0.0052
0.0048
Martensite
area
rate
G
G
F
F
F
F
G
G
G
G
G
P
G
G
G
G
G
P
G
G
G
G
G
P
G
G
G
G
G
Delayed
fracture
characteristic
VG
P
VG
VG
VG
VG
P
P
VG
VG
VG
VG
VG
VG
P
VG
VG
VG
VG
VG
P
VG
VG
VG
VG
VG
P
VG
VG
Oxide
average
particle
size
G
P
G
G
G
G
G
G
G
G
G
G
G
G
P
G
G
G
G
G
P
G
G
G
G
G
P
G
G
Crushed
voids
G
-
G
G
G
G
G
G
G
G
G
G
G
G
-
G
G
G
G
G
-
G
G
G
G
G
-
G
G
Class
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
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S S B I O
9
9
9
9
0
9
3
9
3
9
9
-
9
9
9
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9
9
9
9
9
9
9
9
9
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9
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I
CM
CD
rH
Xi
ro
EH
Crushed Class
voids
Oxide
average
particle
size
Delayed
fracture
characteristic
Martensite
area rate
•H 3
z °
Nb+V+
Co+W
3 o
z
B
o
o
>
z
Steel
sheet
type
Base
steel
type
Steel
type
Ex. no.
Inv. ex.
CJ
o
CJ
>
CJ
0.02 0.01 0.03
os
o
<
t-H
CQ
Ol
Inv. ex.
CJ
CJ
>
CJ
0.01 0.032 0.04
OS
CJ
<
1-D
CQ
CN

CJ
0.5 0.50
OS
o
cC
&S
CQ
Inv. ex.
CJ
CJ
CJ
>
CJ
1.00
T - l
OS
o
cC
i-5
OQ
CTi
Inv. ex.
CJ
CJ
CJ
>
CJ
0.5 0.04 0.54
ns
u
<
s
CQ
LT)
Inv. ex.
CJ
CJ
u
CJ
1.4 0.8 2.20
ctf
O
sC
Z
CQ
cn
Inv. ex.
(J
CJ
Ui
(J
1.5 2.50
rH
g
<
§
CTi
Inv. ex.
cj
CJ
CJ
>
CJ
0.008 0.008
OS
CJ
<
Cu
CQ
03
CXl
Inv. ex.
CJ
CJ
CJ
>
CJ
0.03 0.030
OS
CJ
<
a
CQ
cn
cn
Inv. ex.
CJ
CJ
CJ
>
CJ
0.080
00
o
o
OS
CJ
<
OS
CQ
100
Inv. ex.
cj
CJ
CJ
>
CJ
0.05 0.050
OS
CJ
<
CQ
101
Inv. ex.
CJ
CJ
CJ
>
CJ
0.5 0.500
OS
CJ
<
CQ
102
Inv. ex.
CJ
CJ
CJ
>
(J
0.8 0.800
OS
CJ
a.
ca
£01
Inv. ex.
< j
CJ
(>J
(J
0.03 0.030
OS
CJ
<
>
CQ
104
Inv. ex.
CJ
CJ
CJ
>
CJ
0.02 0.020
cS
CJ
<
CQ
105
Inv. ex.
CJ
CJ
CJ
>
CJ
0.03 0.2 0.230
OS
CJ
<
X
CQ
106
Inv. ex.
cj
CJ
CJ
>
CJ
0.05 0.3 0.350
OS
o
«
>•
0Q
107
Inv. ex.
o
CJ
CJ
>
CJ
0.04 0.03 0.070
os
CJ
<
ISJ
CQ
108
Inv. ex.
CJ
cj
CJ
>
cj
0.08 0.2 0.280
OS
CJ
<
<
CJ
109
Inv. ex.
CJ
CJ
fc4

CJ
0.04 0.3 0.01 0.03 0.340 0.04
OS
CJ
<
CJ
CJ
111
Inv. ex.
CJ
CJ
CJ
>
CJ
0.03 0.340 1.03
t - 0.04 0.3
OS
o
<
a
CJ
112
Inv. ex.
cj
CJ
CJ
>
CJ
0.04 0.3 1.3 0.5 0.340 1.800
OS
o
<
w
CJ
113
Inv. ex.
CJ
CJ
CJ
>
CJ
1.800 0.440 S 'O 1.3 0.1 0.3 0.04
OS
o
<
CJ
rH
rH
Inv. ex.
CJ
cj
CJ
>
CJ
0.1 0.3 0.1 1.3 0.7 0.500 2.000
OS
CJ
<
CJ
CJ
LO
T-H
rH
Inv. ex.
CJ
cj
CJ
>
CJ
0.55 0.3 0.1 0.05 1.3 0.5 1.000 1.800
OS
cj
<
CJ
116
Inv. ex.
CJ
CJ
CJ
>
CJ
0.02 0.01 0.03
OS
CJ
X
M
O
117
Inv. ex.
CJ
o
CJ >
u
0.01 0.032 0.04
OS
CJ
X
1-D
CJ
118
Inv. ex.
CJ
CJ
CJ
>
CJ
0.5 0.50
OS
u
X
o
611
Inv. ex.
CJ
CJ
CJ >
CJ
1.00
-
s
X
d
120
Inv. ex.
CJ
CJ
CJ
>
CJ
0.5 0.04 0.54
OS o
X
2
CJ
121
Inv. ex.
CJ
CJ
[ u
o
1.4 0.8 2.20
os
o
X
z
CJ
122
Table 2-2 (mass%)
Ex. no.
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
Steel
type
CO
CP
CQ
CR
CS
CT
CU
CV
CW
CX
CY
CZ
DA
DB
DC
DD
DE
DF
DG
DH
DC
DD
DE
DF
DG
DH
DC
DD
DE
DF
DG
DH
DC
Base
steel
type
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Steel
sheet
type
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
AL
AL
AL
AL
AL
AL
GI
GI
GI
GI
GI
GI
GA
Nb
0.005
0.032
0.081
0.03
0.048
0.04
0.08
0.09
0.05
0.05
0.05
0.05
0.15
0.55
0.05
0.05
0.05
0.05
0.15
0.55
0.05
0.05
0.05
0.05
0.15
0.55
0.05
V
0.053
0.48
0.79
0.2
0.3
0.5
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
Co
0.02
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
W
0.03
0.03
0.2
0.8
0.05
0.05
0.05
Ni
1
0.01
1
1.3
1.3
1.3
1.3
0.01
1
1.3
1.3
1.3
1.3
0.01
1
1.3
1.3
1.3
1.3
0.01
Cu
1.5
0.03
0.03
0.5
0.5
0.7
0.5
0.03
0.03
0.5
0.5
0.7
0.5
0.03
0.03
0.5
0.5
0.7
0.5
0.03
Nb+V+
Co+W
0.005
0.032
0.081
0.053
0.480
0.790
0.030
0.020
0.230
0.348
0.070
0.280
1.490
0.350
0.350
0.350
0.450
0.550
1.000
0.350
0.350
0.350
0.450
0.550
1.000
0.350
0.350
0.350
0.450
0.550
1.000
0.350
Ni +
Cu
2.50
0.04
1.03
1.800
1.800
2.000
1.800
0.04
1.03
1.800
1.800
2.000
1.800
0.04
1.03
1.800
1.800
2.000
1.800
0.04
Martensite
area rate
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
Delayed
fracture
characteristic
F
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
F
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
VG
Oxide
average
particle
size
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
Crushed
voids
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
Class
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
I
mmw£immmmmi!v,m??<®m.
^
- 32 -
o\o
CO
CO
e
i
CM
fO
Crushed Class
voids
Oxide
average
particle
size
Delayed
fracture
characteristic
Martensite
area rate
•H 3
> s
+ +
na °°
3
U
•H
s
o
o
>
X!
2
Steel
sheet
type
Base
steel
type
Steel
type
Ex. no.
Inv. ex.
co
15
co
>
co
0.03 0.350 1.03
TH
0.05 0.3
co
X
p
p
156
Inv. ex.
co
co
co >
co
1.800 0.350 0.5 1.3 0.3 S O 'O
<
co
X
M
P
157
Inv. ex.
to
co
15
>
co
0.05 0.3 0.1 1.3 0.5 0.450 1.800
<*
C5
X
Pu
p
158
Inv. ex.
to
co
>
co
2.000 0.550 0.7 1.3 0.1 0.3 0.15 <
co
X
p
159
Inv. ex.
to
co
ID
>
CD
1.800 1.000 0.5 1.3 SO'O 0.1 0.3 0.55
ft
cs
X
as
p
160
Inv. ex.
to
CD
IS
>
CD
0.02 0.01 0.03
g
S
M
P
161
Inv. ex.
CD
co
co >
co
0.01 0.032 0.04
OS
o
CJ <
1-D
P
162
Inv. ex.
co
CD
co >
o
0.5 0.50
os
o
<
p
163
Inv. ex.
co
co
>
co
1.00
.-H
OS
o
CJ
p
164
Inv. ex.
co
CO
to >
co
0.5 0.04 0.54
OS
o
<
s p
165
Inv. ex.
co
co
fcy
co
2.20
CD
O
1.4
OS
o
<
S3
p
166
Inv. ex.
co
o
[u
co
1.5 2.50
tH
OS
o
<
o
p
167
Inv. ex.
to
co
>
to
0.005 0.005
os
o
<
Oj
p
168
Inv. ex.
co
co
>
co
0.032 0.032
os
o
CJ
O
p
169
Inv. ex.
co
o
to >
co
0.081 0.081
OS
CJ
u
OS
p
170
Inv. ex.
CD
co
co
>
to
0.053 0.053
OS
o
CJ <
CO
p
171
Inv. ex.
co
e>
co
>
(j
0.48 0.480
OS
o
u <
EH
P
172
Inv. ex.
CD
co
co
>
d
0.79 0.790
os
o
u
ft
P
173
Inv. ex.
15
co
co >
O
0.03 0.030
OS
cj
CJ
>
174
Inv. ex.
co
co
>
to
0.02 0.020
OS
CJ
s
p
175
Inv. ex.
CD
co
to >
to
0.03 0.2 0.230
OS
o
<
X
p
176
Inv. ex.
to
to
to >
to
0.048 0.3 0.348
os
o
CJ
ft
>H
P
177
Inv. ex.
to
to
to >
to
0.04 0.03 0.070
OS
CJ
o
ft
CS1
p
178
Inv. ex.
to
to
to >
to
0.08 0.2 0.280
OS
o
CJ
ft
ft w
179
Inv. ex.
CO
to
Cu
CO
1.490
CD
O
0.09 0.5 0.1
OS
CJ
CJ
<
03
[d
180
Inv. ex.
CO
CO
CO
>
CO
0.05 0.3 0.01 0.03 0.350 0.04
os
CJ
o
ft
CJ
Id
181
Inv. ex.
CO
to
to >
CO
0.03 0.350 1.03
rH
0.05 " 0.3
os
CJ
u
ft
p w
182
Inv. ex.
to
CO
CO
>
CO
1.800 0.350 0.5 E 'T 0.3 0.05
OS
CJ
CJ
<
Ed
183
Inv. ex.
CO
CO
CO
>
CO
0.05 0.3 0.1 1.3 0.5 0.450 1.800
OS
o
CJ
ft
CO
td
184
Inv. ex.
to
CO
CO
>
to
0.15 0.3 0.1 1.3 0.7 0.550 2.000
os
o
CJ
ft
as
td
185
Inv. ex.
CO
CO
to >
CO
0.55 0.3^ 0.1 0.05 1.3 0.5 1.000 1.800
os
CJ
CJ
ft
H
186
- 33 -
[0083] Example 2
The Steel Types A, X, and AC which are shown in Tables 1-
1 and 1-2 were used to study the rolling conditions.
These slabs were heated to 1050 to 1350°C then hot rolled
5 by a finish temperature of 800 to 900°C and a coiling
temperature of 450 to 680°C to obtain hot rolled steel
sheets. The slabs, rough rolled sheets, the thickness and
rough rolling rate of the hot rolled sheets, and the
final rolling rate are shown in Tables 3-1 and 3-2. After
10 that, part of the hot rolled steel sheets were pickled,
then cold rolled. The cold rolled sheet thickness and
cold rolling rate are shown in Tables 3-1 and 3-2. After
that, part of the steel sheets was continuously annealed
(annealing temperature 720 to 830°C). Further, parts of
15 the steel sheets were hot dip galvanized (basis weight:
one side 30 to 90 g/m2) , hot dip galvannealed (basis
weight: one side 30 to 90 g/m2), and hot dip aluminum
coated (basis weight: one side 30 to 100 g/m2) on a
continuous hot dipping line. The steel sheet types are
20 shown in Tables 3. The types of steel sheets are shown
below:
HR: hot rolled steel sheet, CR: cold rolled steel sheet
(annealed material), AL: hot dip aluminum coated steel
sheet, GI: hot dip galvanized steel sheet, and GA: hot
25 dip galvannealed steel sheet.
[0084] The average particle size of the Fe-Mn
composite oxides in a produced steel sheet and the
presence of crushed voids were determined by polishing a
cross-section of the steel sheet, then observing it by an
30 optical microscope or scan type electron microscope or by
a transmission type electron microscope after preparing
the sample into a thin film. The results are shown in
Tables 3-1 to 3-2. The judgment criteria are shown below:
Average particle size of composite oxides:
35 Good: average diameter 0.1 to 15 (j,m,
Poor: average diameter less than 0.1 urn or over 15 (j.m
- 34 -
Crushed voids around composite oxides:
Good: average size of voids 0.1 fj,m or more
Poor: average size of voids less than 0.1 \xm
[0085] After that, these cold rolled steel sheets were
5 heated by a heating furnace to over the Ac3 point, that
is, the 880 to 950°C austenite region, then were hot
worked. For the atmosphere of the heating furnace,
combustion exhaust gas was used. The hydrogen
concentration in the atmosphere was 2%, while the dew-
10 point was 20°C.
[0086] The cross-section of the shape of the die set
which is used in the examples is shown in FIG. 4. FIG. 4
shows the shapes of the die 9 and punch 10. The shape of
the punch as seen from above is shown in FIG. 5. FIG. 5
15 shows the punch 10. The shape of the die as seen from
below is shown in FIG. 6. FIG. 6 shows the die 9. In the
die set, the shape of the die is determined based on the
punch with a clearance of the sheet thickness of 1.6 mm.
The blank size was made 1.6 mm thicknessx300 mmx500 mm.
20 The shaping conditions were made a punch speed of 10
mm/s, a pressing force of 200 tons, and a holding time
at bottom dead center of 5 seconds. A schematic view of
the hot pressed part is shown in FIG. 7.
[0087] The quenching characteristic of the steel sheet
25 was evaluated by polishing the cross-section, corroding
it by Nital, then observing the microstructure by an
optical microscope and determining the area rate of
martensite. The results of judgment are shown in Tables
3-1 and 3-2. The judgment criteria are shown below:
30 Good: martensite area rate 90% or more,
Fair: martensite area rate 80% or more, and
Poor: martensite area rate less than 80%.
[0088] The delayed fracture resistance was evaluated
by imparting stress by piercing. The pierce hole position
35 13 at the center of the test part 12 which is shown in
FIG. 8 was pierced using a diameter 10 mm punch and using
* - 35 -
a die of a diameter giving a clearance 15%±2. FIG. 8
shows the shape of the part seen from above. FIG. 8 shows
the part 12 and the pierce hole center 13. The piercing
was performed within 30 minutes after hot shaping. The
5 number of parts observed was 10. For judgment of the
hydrogen embrittlement resistance, the entire
circumference of the hole was observed one week after
piercing to judge the presence of any cracks. The state
was observed by a loupe or electron microscope. The
10 results of judgment are shown in Tables 3-1 and 3-2. The
judgment criteria are shown below:
Total of number of parts with fine cracks in 10 parts:
Very good: 0,
Good: 1,
15 Fair: less than 5, and
Poor: 5 or more.
[0089] As shown in Tables 3-1 and 3-2, it is learned
that if in the scope of the method of production which is
recommended by the present invention, steel sheet can be
20 realized which is sufficiently strengthened by die
quenching by hot stamping and which is more excellent in
delayed fracture resistance.
- 36
Class
Crushed
void
Oxide
average
particle
size
Delayed
fracture
rate
Martensite
area rate
Cold
rolling
rate
{%)
Final
rolling
rate
(%)
Rough
rolling
rate
(%)
Cold
rolling
thickness
(mm)
Hot
rolling
thickness
(mm)
Rough
rolling
thickness
(mm)
Slab
thickness
(mm)
Steel
sheet
type
Rolling
Steel
type
Ex. no.
Comp. ex.
CU
Cu
CU
o
67.5
CN
6.5
o
CN
250
Z
05
35
<
187
Inv. ex.
o
o
o
o
70.0
CN
cn
CD
o
CN
250 a
05
35
<
187-1
Inv. ex.
a
o
tu
CJ
78.3
CO
CO
6.5
o
cn
250
a
05
X
<
188
Inv. ex.
a
o
u
o
83.8
00
6.5
o
250
a
05
X
<
189
Comp. ex.
Cu
On
OJ
O
92.5
CD
CD
oo
o
100
a
05
35
<
190
Inv. ex.
o
o
[ u
o
73.333 92.5
cn
o
150
a
05
35
KC
191
Inv. ex.
u
o
Pu
o
73.333 92.5
CO
o
150
• J
<:
05
35
<
192
Inv. ex.
o
o
to
o
73.333 92.5
CO
o
150
M
05
35
<
193
Inv. ex.
o
o
t<
o
73.333 92.5
CO
o
150
05
35
<
194
Inv. ex.
cs
u
e>
e>
92.5
o
CO
CO
o
200
a
05
35
rt
195
Inv. ex.
u
u
o
e>
92.5
00
CO
o 250
a
05
35
<
196
Comp. ex.
o
cu
Cu
u
96.3 20.0
00
1.5 1.2
o
250
a
05
o
<
197
Inv. ex.
o
o
ta
e>
95.3 36.8
CO
1.9 1.2
CD
250
a
05
o
<
198
Inv. ex.
o
o
u
u
93.8 52.0
CO
2.5 1.2
CD
250
a
05
o
<
199
Inv. ex.
u
u
u
u
93.8 52.0
00
2.5 1.2
o
250

o
o
u
93.8 52.0
00
2.5 1.2
o
250
05
O
<
202
Inv. ex.
is
u
is
o
93.8 52.0
CO
2.5 1.2
o
•31
250
05
o
a.
203
Inv. ex.
o
o
>
o
92.5 60.0
CO
1.2
CO
o
250
25
05 o
rt
204
Inv. ex.
o
o
>
u
90.0 70.0
00
1.2
"C
o
250
a
05
<
205
Inv. ex.
o
o
>
o
87.5 76.0
00
1.2
i n
o
250
a
05
o
<
90S
Comp. ex.
cu
Cu
Cu
e>
67.5
CM
CTi
6.5
o
CM
250
a
05
35
X
207
Inv. ex.
o
o
u
u
70.0
CN
cn
C£>
O
CN
250
a
05
35
X
207-1
Inv. ex.
u
u
tu
u
78.3
00
00
6.5
CD
CO
250
a
05
35
X
80S
Inv. ex.
u
u
o
13
83.8 CO 6.5
o
250
a
05
35
X
60S
Comp. ex.
cu
cu
Cu
O
92.5
o
CD
CO
CD
100
a
05
35
X
210
Inv. ex.
o
o
UA
a
73.333 92. 5
CO
o
150
a
05
35
X
TTS
Inv. ex.
o
o
o
d
92.5
o
00
CO
o
200
a
05
35
X
212
Inv. ex.
e>
o
o
o
92.5
CO
CO
o
250
S
Pi
X
213
Inv. ex.
o
o
u
u
92.5
00
CO
o
250
<
05
35
X
214
Inv. ex.
o
u
u
u
92.5
oo
CO
o
250
M
05
35
X
215
I'
Table 3-2
Ex. no.
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
Steel
type
X
X
X
X
X
X
X
X
X
X
X
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
AC
Rolling
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
HR
HR
HR
HR
HR
HR
HR
HR
HR
HR
CR
CR
CR
CR
CR
CR
CR
CR
CR
CR
Steel
sheet
type
GA
N
N
N
N
N
A
AL
GI
GA
N
N
N
N
N
N
N
AL
GI
GA
N
N
N
A
AL
GI
GA
N
N
N
N
Slab
thickness
(mm)
250
250
250
250
250
250
250
250
250
250
250
250
250
250
100
150
200
200
200
200
250
250
250
250
250
250
250
250
250
250
250
Rough
rolling
thickness
(mm)
40
40
40
40
40
40
40
40
40
40
40
20
30
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
40
Hot
rolling
thickness
(mm)
3
1.5
1.9
2.5
3
4
4
4
4
4
5
6.5
6.5
6.5
3
3
3
3
3
3
3
1.5
1.9
1.9
1.9
1.9
1.9
2.5
3
4
5
Cold
rolling
thickness
(mm)
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
Rough
rolling
rate
(%)
84
84
84
84
84
84
84
84
84
84
84
92
88
84
60
73.333
80
80
80
80
84
84
84
84
84
84
84
84
84
84
84
Final
rolling
rate
(%)
92.5
96.3
95.3
93.8
92.5
90.0
90.0
90.0
90.0
90.0
87.5
67.5
78.3
83.8
92.5
92.5
92.5
92.5
92.5
92.5
92.5
96.3
95.3
95.3
95.3
95.3
95.3
93.8
92.5
90.0
87.5
Cold
rolling
rate
(%)
20.0
36.8
52.0
60.0
70.0
70.0
70.0
70.0
70.0
76.0
20.0
36.8
36.8
36.8
36.8
36.8
52.0
60.0
70.0
76.0
Martensite
area rate
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
Delayed
fracture
rate
G
P
F
G
VG
VG
G
G
G
G
VG
P
F
G
P
F
G
G
G
G
G
P
F
F
F
F
F
G
VG
VG
VG
Oxide
average
particle
size
G
P
G
G
G
G
G
G
G
G
G
P
G
G
P
G
G
G
G
G
G
P
G
G
G
G
G
G
G
G
G
Crushed
void
G
G
G
G
G
G
G
G
G
G
G
P
G
G
P
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
G
Class
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Comp. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
Inv. ex.
- 38 -
Industrial Applicability
[0091] The present invention can be used as a steel
material for hot stamping use. Regarding its field of
use, this can be utilized in a broad range of industrial
5 fields such as auto parts, home electrical appliances,
machinery, etc.
Reference Signs List
[0092] 1 coarse composite oxides
10 1-1, 1-2 oxides
2 hot rough rolling
3 stretched composite oxides
3-1, 3-2 stretched oxides
4 hot final rolling
15 5 crushed void (hydrogen trapping ability)
5-1 and 5-2 crushed oxides
6 coarse oxides
6' fine oxides
7 stretched oxides
20 8 crushed oxides
9 die
10 punch
11 hot stamped part
12 test part
25 13 pierced hole position

- 39 -
CLAIMS
Claim 1. Steel sheet for hot stamping use which is
comprised of chemical ingredients which contain, by
mass%,
5 C: 0.05 to 0.40%,
Si: 0.001 to 0.02%,
Mn: 0.1 to 3%,
Al: 0.0002 to 0.005%,
Ti: 0.0005 to 0.01%,
10 0: 0.003 to 0.03%,
one or more of Cr and Mo in a total of 0.005 to
2%, and
a balance of Fe and unavoidable impurities,
wherein the steel sheet contains average
15 diameter 0.1 to 15 |um Fe-Mn-based composite oxide
particles dispersed in the steel sheet.
Claim 2. The steel sheet for hot stamping use as
set forth in claim 1 which further contains, by mass%,
the ingredients which are included in one or more groups
20 among the three groups of (a) to (c):
(a) B: 0.0005 to 0.01%;
(b) one or more of Nb, V, W, and Co in a total of
0.005 to 1%; and
(c) one or more of Ni and Cu in a total of 0.005 to
2 5 Z "t> .
Claim 3. The steel sheet for hot stamping use as
set forth in claim 1 or 2, wherein there are voids around
said composite oxide particles.
Claim 4. The steel sheet for hot stamping use as
30 set forth in claim 1 or 2, wherein the voids around said
composite oxide particles have average sizes of 10 to
100% of the average size of said composite oxide
particles.
Claim 5. The steel sheet for hot stamping use as
35 set forth in claim 1 or 2, wherein the steel sheet is
plated by any of aluminum plating, zinc-aluminum plating,
and zinc plating.
- 40 -
Claim 6. A method of production of steel sheet for
hot stamping use comprising hot rolling a slab of
chemical ingredients set forth in claim 1 or 2 in which
rough rolling the slab by a rolling rate of 70% or more
5 and final rolling the slab by a rolling rate of 70% or
more.
Claim 7. The method of production of steel sheet
for hot stamping use as set forth in claim 6, further
comprising pickling the hot rolled steel sheet which was
10 obtained by hot rolling and cold rolling the steel sheet
by a rolling rate of 30% or more.
Claim 8. The method of production of steel sheet
for hot stamping use as set forth in claim 7, further
comprising annealing the cold rolled steel sheet which
15 was obtained by cold rolling.
Claim 9. A method of production of a high strength
part using the steel sheet for hot stamping use
comprising heating the steel sheet as set forth in claim
1 or 2 to a temperature of austenite region of the AC3 or
20 higher, then starting to form the steel sheet by a die,
and cooling the steel sheet in the die after forming to
quench.